The disposal of soils and industrial wastes contaminated with mercury or mercury compounds has created major environmental and economic problems for U.S. industries. The U.S. Environmental Protection Agency (EPA) has classified any industrial waste product having a leachable mercury content of 0.2 parts per million (ppm) or higher (as measured by the standard EPA TCLP Leach Test) as hazardous waste that must be disposed of in a secure hazardous waste land disposal facility. In addition, many state environmental regulatory agencies are requiring that soils contaminated with mercury must be treated to reduce the mercury content to levels equivalent to the background levels that existed in the soils prior to contamination before the treated soils can be disposed of or used as land fill material.
The relatively low mercury content and large volume of inert materials in contaminated soils and industrial wastes make economic recovery of the contained mercury difficult and the disposal of the contaminated waste extremely costly. As a result, there is a need for a method and apparatus for removing mercury form soils and industrial wastes. Such method and apparatus must reduce the mercury content of the soils and industrial wastes to at least the background level, must reduce the mercury content of the soils and industrial wastes to 0.2 ppm or less and must recover the removed mercury in usable form in order to recover some economic value and breaking the chain of liability inherent in the disposal of mercury-containing wastes classified as hazardous by the EPA.
Mercury contamination of soil is especially acute in the areas of metering station sites along natural gas pipelines where, for over a quarter of a century, metallic mercury was routinely discharged onto the ground each time the mercury used in flow measuring devices was replaced or upon the failure of the devices. These contaminated sites are characterized by relatively low concentrations of mercury (from about 100 ppm to about 2,000 ppm). These sites also have substantial variations in soil constituents (such as clay minerals, iron and/or manganese oxides and hydroxides, gypsum, jarosite, quartz, carbonates, organic and carbonaceous materials and the like). Further, the mercury is present in a wide variety of forms (such metallic mercury, mercury oxides and sulfides, methyl mercury and the like). These site characteristics, individually and collectively, have previously prevented the efficient, complete and economic removal of the mercury contamination from the soil at these sites.
The efficient and economical decontamination of such sites requires a mercury removal process that is capable of processing soils that vary substantially in type and composition, and, simultaneously, removes the wide variety of forms of mercury that may exist in the soil. Such a system should reduce the mercury content of the treated soil to the background level of mercury (usually less than about 1 ppm) normally present in that particular soil. The process should also permit recovery of the removed mercury in a reusable form. The equipment associated with method should be sufficiently portable to be economically moved from site to site, thereby elimination the need to transport large volumes of soil to a remote treatment facility in order to remove the mercury from the soil.
Similar problems are encountered when treating mercury-containing industrial and incinerator wastes which also vary greatly in mineral form and composition from site to site, contain relatively small amounts of mercury and are located over a wide geographic area in quantities too small to permit the economic recovery of the mercury contained therein.
Current methods and techniques for stabilizing and encapsulating mercury species contained in soils and industrial wastes are capable of significantly reducing the leachablility of mercury from those soils and wastes but at the cost of greatly increasing the volume of the materials requiring disposal. In addition, the current methods and techniques do not reduce the mercury content of the treated product and thereby extend the chain of environmental liability related to the disposal or reuse of the treated soils or wastes.
None of the prior art methods or apparatus are adaptable to the economical, on-site reduction of the leachable content of mercury in the mercury-contaminated soils and industrial wastes. Nor do the methods or apparatus convert mercury-contaminated materials classified as "hazardous waste" to materials classified as "non-hazardous waste." Further, none of the prior art methods or apparatus recover the removed mercury in a usable form thereby ending the environmental liability related to treated soils or wastes.
There remains, therefore, a need for a practical method of (a) economically removing mercury from contaminated soils and industrial wastes that vary greatly in composition, physical form, and the species of mercury contained therein; (b) on-site conversion of mercury-containing soils and industrial wastes classified as "hazardous" to conversion products classified as "non-hazardous" by the removal of the mercury therefrom; (c) recovering the removed mercury in usable form; and (d) thermally treating the mercury-containing soils and industrial wastes in a manner that permits control of the mercury, sulfur, dust, and organic contents of the gaseous process effluent to a degree that the effluent is considered harmless to environment and to persons practicing the method. Such a system incorporating these improvements would substantially reduce the cost of treating mercury-contaminated soils and industrial wastes and greatly reduce the environmental dangers inherent therein.
It is, therefore, an object of the present invention to provide a method and apparatus for removing mercury and mercury compounds from mercury-contaminated soils and industrial wastes.
It is another object of the present invention to provide such a system that reduces the mercury content of mercury-contaminated soils to a level no greater than the background level of mercury contained in the soil prior to the contamination.
It is a further object of this invention to reduce the leachable mercury content of mercury-containing industrial wastes to a level of less than about 0.2 ppm as measured by the standard EPA TCLP Leach Test.
It is yet another object of this invention to minimize the content of gaseous sulfur compounds generated during the thermal treatment of mercury-contaminated soils and industrial wastes.
It is also an object of the present invention to maximize the amount of elemental mercury recovered during the thermal treatment of mercury-contaminated soils and industrial wastes.
It is yet another object of the present invention to provide a transportable means of thermally treating mercury-contaminated soils and industrial wastes such that the means may be economically and efficiently transported to various sites at which the mercury-contaminated soils and industrial wastes are located.
Consideration of the specification, including the several figures to follow, will enable one skilled in the art to determine additional objects and advantages of the invention.